Field of the Invention
This invention relates to methods for starting fuel cells from temperatures below 0° C. In particular, it relates to methods for preventing ice formation during startup.
Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising fluid distribution channels are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is low (of order of 1V), a plurality of cells are usually stacked together in series for commercial applications. And fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
In certain applications, PEMFC stacks may be subjected to repeated on-off duty cycles involving storage for varied lengths of time and at varied temperatures. It is generally desirable to be able to reliably startup such stacks in a short period of time. Certain applications, like automotive, can require relatively rapid reliable startup from storage conditions well below freezing. This has posed a significant challenge both because of the relatively low rate capability of cells at such temperatures and also because of problems associated with water management in the cells when operating below 0° C. A certain amount of water is required for proper fuel cell operation (e.g. hydration of the membrane electrolyte) and is generated as a result of providing electrical power. However, ice of course forms where liquid water is present at such temperatures. The presence of ice can be problematic depending on how much there is and its location when stored or when starting up.
Various fuel cell designs and startup methods have been developed in the art to provide for improved startup from temperatures below freezing. For instance, US patent application serial numbers 20050053809 and 20060141309 both teach using greater oxidant flow rates than usual when starting up from temperatures below 0° C. Also, US patent application serial number 20010028967 teaches various methods employing reactant starvation to provide for improved startup. These methods are typically intermittent and involve starving the cell of reactant (i.e. where reactant stoichiometry is less than 1). Further, US patent application serial number 20060134472 teaches methods for operating a stack such that relatively dry conditions are maintained therein prior to storage. This can improve subsequent startup but may involve trading off optimum performance capability prior to shutdown.
Despite the advances made to date, there remains a need for more rapid, reliable startup methods for PEMFCs under all the operating conditions they may normally be expected to encounter. This invention fulfils these needs and provides further related advantages.